Patent application title: DATE TRANSMISSION DEVICE

Abstract:

A data transmission device for sending back data on a reflected wave of an
unmodulated carrier, the data transmission device includes: an antenna
adapted to receive an unmodulated carrier Fo from a data reader with
which data is to be exchanged and send a reflected wave of the
unmodulated carrier Fo to the data reader; subcarrier oscillation means
for generating a subcarrier frequency Fs; subcarrier modulation means for
digitally modulating sending data on the subcarrier Fs; D/A conversion
means for generating the analog modulated subcarrier wave Fs by
converting a digital modulated signal, generated by the subcarrier
modulation means, into analog form; and frequency mixing means for mixing
the unmodulated carrier Fo and modulated subcarrier wave Fs to generate
new modulated waves Fo+Fs and Fo-Fs.

Claims:

1. A data transmission device for sending back data on a reflected wave of
an unmodulated carrier, the data transmission device comprising:an
antenna adapted to receive an unmodulated carrier Fo from a data reader
with which data is to be exchanged and send a reflected wave of the
unmodulated carrier Fo to the data reader;subcarrier oscillation means
for generating a subcarrier frequency Fs;subcarrier modulation means for
digitally modulating sending data on the subcarrier Fs;Digital/Analog
conversion means for generating the analog modulated subcarrier wave Fs
by converting a digital modulated signal, generated by the subcarrier
modulation means, into analog form; andfrequency mixing means for mixing
the unmodulated carrier Fo and modulated subcarrier wave Fs to generate
new modulated waves Fo+Fs and Fo-Fs.

4. The data transmission device of claim 1, the frequency mixing means are
an analog mixer which includes a diode set to operate in the
non-saturated region, the analog mixer having two ports adapted to
receive an analog modulated signal from the Digital/Analog conversion
means and an antenna-received signal received via the band-pass filter,
whereinthe band-pass filter is provided between the antenna and frequency
mixing means to pass Fo and one of Fo+Fs and Fo-Fs.

5. The data transmission device of claim 1, the frequency mixing means
comprising:a splitter adapted to divide the antenna-received signal;an
Field Effect Transistor mixer adapted to mix the analog modulated wave Fs
and antenna-received signal Fo divided by the splitter to generate
modulated reflected waves, namely, new modulated waves Fo+Fs and Fo-Fs;
anda band-pass filter having the property to pass only one of the
modulated reflected waves Fo+Fs and Fo-Fs, whereinthe splitter transmits
one of the modulated reflected waves Fo+Fs and Fo-Fs from the antenna.

7. The data transmission device of claim 1 further comprising:backscatter
modulation means which rely on the change of an antenna load impedance
caused by an on/off operation of the antenna load selector switch to
modulate the reflected wave of the antenna-received radio wave, the
on/off operation of the switch being made according to a digital
modulated signal generated by the subcarrier modulation means; andan
antenna switch adapted to connect the frequency mixing means or
backscatter modulation means to the antenna according to the modulation
scheme used by the subcarrier modulation means,the backscatter modulation
means includingan antenna load selector switch adapted to change the load
impedance of the antenna; anda band-pass filter disposed between the
antenna and antenna load selector switch, the band-pass filter having the
property to pass the frequency Fo and one of the modulated reflected
waves at the frequencies Fo+Fs and Fo-Fs.

8. The data transmission device of claim 7, the subcarrier modulation
means can use, according to the communication quality, either a
modulation scheme such as Phase Shift Keying, Quadrature Phase Shift
Keying or 8 Phase Shift Keying which demands only binary amplitude
information, or a modulation scheme such as 16 Quadrature Amplitude
Modulation, 64 Quadrature Amplitude Modulation or Orthogonal Frequency
Division Multiplexing which carries binary or multi-level amplitude
information, whereinthe antenna switch connects the backscatter
modulation means to the antenna when a modulation scheme, which demands
only binary amplitude information, is used by the subcarrier modulation
means, and connects the frequency mixing means to the antenna when a
modulation scheme carrying binary or multi-level amplitude information is
used by the subcarrier modulation means.

9. A data transmission device for sending back data on a reflected wave of
an unmodulated carrier, the data transmission device comprising:an
antenna adapted to receive an unmodulated carrier Fo from a data reader
with which data is to be exchanged and send a reflected wave of the
unmodulated carrier Fo to the data reader;a subcarrier oscillation unit
adapted to generate a subcarrier frequency Fs;a subcarrier modulation
unit adapted to digitally modulate sending data on the subcarrier Fs;a
Digital/Analog conversion unit adapted to generate the analog modulated
subcarrier wave Fs by converting a digital modulated signal, generated by
the subcarrier modulation means, into analog form; anda frequency mixing
unit adapted to mix the unmodulated carrier Fo and modulated subcarrier
wave Fs to generate new modulated waves Fo+Fs and Fo-Fs.

Description:

CROSS REFERENCES TO RELATED APPLICATIONS

[0001]The present application claims priority related to Japanese Patent
Application JP 2007-066184 filed with the Japan Patent Office on Mar. 15,
2007, the entire contents of which is being incorporated herein by
reference.

BACKGROUND

[0002]The present application relates to a data transmission device of a
reflected wave transmission system for transmitting data using a
modulated reflected wave generated by modulating an unmodulated carrier
supplied from a data reader. The present application relates more
particularly to a data transmission device for handling high speed data
transmission by performing a primary modulation of a subcarrier followed
by a secondary modulation of a reflected wave.

[0003]More specifically, the present application relates to a data
transmission device for providing faster reflected wave data transmission
by using a primary modulation which can carry binary or multi-level
amplitude information. The present application relates more particularly
to a data transmission device for suppressing a modulated reflected wave
from spreading into side lobes during a primary modulation of a
subcarrier.

[0004]A non-contact communication system called RFID (Radio Frequency
Identification) is known as a communication system adapted to wirelessly
send data without having any own radio wave generating source. The RFID
includes a tag and reader. The tag is a passive device, receiving radio
wave from the reader as an energy source so that information is read from
the tag.

[0005]Although referred to in various manners such as ID System and Data
Carrier System, the RFID System or RFID for short, is a globally common
name. The RFID can be translated to mean Identification System Using High
Frequency (Radio Frequency) in Japanese.

[0006]Non-contact communication techniques used for the RFID system are
capacitive coupling, electromagnetic induction and radio communication.
Of these techniques, the RFID system based on radio communication
includes a reflector and reflected wave reader. The reflector sends data
on a reflected wave generated by modulating a received unmodulated
carrier. The reflected wave reader reads data from the modulated
reflected wave from the reflector. This system performs reflected wave
transmission called "backscatter."

[0007]Upon receipt of an unmodulated carrier from the reflected wave
reader, the reflector modulates data on the reflected wave of the
carrier, for example, by changing the antenna load impedance. That is,
the reflector demands no carrier generating source. This makes it
possible for the reflector to ensure low power consumption in data
transmission. Upon receipt of such a modulated reflected wave, the
reflected wave reader demodulates and decodes the received reflected wave
to obtain transmitted data.

[0008]On the other hand, an antenna switch adapted to change the antenna
load impedance of the reflector may be incorporated in a circuit module
and configured with CMOS (Complementary Metal Oxide Semiconductor)
transistors. However, the switch provides low power consumption and fast
switching if it is separate from the circuit module and configured with
gallium arsenide (GaAs) IC (Integrated Circuit). The latter offers
improved data transmission rate using reflected wave modulation and keeps
the power consumption to several tens of μW or less. Considering that
wireless LAN consumes several hundreds of mW to several W during
communication, reflected wave communication can be said to be by far
superior in performance to typical wireless LAN in terms of average power
consumption (refer, for example, to Japanese Patent Laid-Open No. Hei
10-209914, hereinafter referred to as Patent Document 2).

[0009]A reflector-equipped terminal does nothing but reflect the received
radio wave. As a result, the terminal is not regarded as a radio station.
Instead, it is treated as a device not subject to laws or regulations
relating to radio wave communication. Further, other types of non-contact
communication systems such as those based on electromagnetic induction
employ frequencies between several MHz to several hundreds of MHz (e.g.,
13.65 MHz). In contrast, systems based on reflected wave communication
can provide high speed data transmission using, for example, the high
frequency 2.4 GHz (microwave) band, which is referred to as the ISM
(Industry, Science, and Medical Band).

[0010]For example, a reflector is built into a terminal device whose power
consumption should be kept to a minimum such as digital camera, video
camcorder, mobile phone, mobile information terminal, or portable music
player. A reflected wave reader is built into host equipment which
includes a stationary household electric appliance such as television
set, monitor, printer, PC, VCR, or DVD player. This makes it possible to
upload image data captured with a camera-equipped mobile phone or digital
camera to the PC via a reflected wave transmission line so that such
image data can be accumulated, displayed or printed.

[0011]In reflected wave transmission, the frequency of the carrier from
the reader is normally the same as the center frequency of the reflected
wave. This forces the reader to handle sending and reception at the same
frequency. As a result, the receiving section is prone to DC offset and
sender noise due to coupling loop interference caused by a sending
signal. This makes it difficult to expand the transmission distance.
Therefore, the isolation between the sender and receiver is a problem to
be addressed. On the other hand, the reflected wave transmission employs
ASK (Amplitude Shift Keying) or PSK (Phase Shift Keying) for modulation
in almost all cases, making it difficult to provide faster transmission.
A solution proposed to solve these problems is to modulate data on a
subcarrier, for example, by PSK, QPSK or 8PSK as a primary modulation,
followed by modulation of the reflected wave by changing the antenna load
impedance using an antenna switch (refer, for example, to Patent Document
2).

[0012]In the above reflected wave transmission adapted to perform a
primary modulation of a subcarrier, however, the antenna switch is turned
on and off using a binary digital signal carried by the subcarrier. As a
result, only relatively slow subcarrier modulation schemes, such as ASK,
PSK, QPSK (Quadrature PSK) and 8PSK, which demand only binary amplitude
information, can be used. In other words, modulation schemes carrying
binary or multi-level amplitude information such as 16QAM (Quadrature
Amplitude Modulation), 64QAM and OFDM (Orthogonal Frequency Division
Multiplexing) cannot be used.

[0013]Further, even when a modulation scheme such as ASK, PSK, QPSK or
8PSK is used, it is impossible to modulate the reflected wave using the
antenna switch if the digital modulated signal is limited in bandwidth
and converted into analog form. In this case, the frequency spectrum of
the reflected wave spreads out infinitely, raising concern that other
systems in the neighborhood may be affected by interference. These
problems will be described below.

[0014]FIG. 6 illustrates a configuration example of a reflected wave
transmission system using a subcarrier. In FIG. 6, reference numeral 10
denotes a data transmission device adapted to send data by modulating the
reflected wave of a received radio wave. Reference numeral 11 denotes a
data reader adapted to receive the modulated reflected wave signal from
the transmission device 10 to read data therefrom.

[0015]The data transmission device 10 is incorporated in a mobile device
which serves primarily as a data transmission source such as digital
camera or mobile phone. The same device 10 transmits moving image data
and music data stored therein to the reader 11. As illustrated in FIG. 6,
the transmission device 10 includes an antenna 100, a band-pass filter
(BPF) 101, an antenna load selector switch 102, a subcarrier oscillator
103 having a frequency Fs and a subcarrier modulator 104.

[0016]One end of the antenna load selector switch 102 is grounded. The
same switch 102 serves as a load of the antenna 102 and is
short-circuited when turned on and open-circuited when turned off, thus
allowing an unmodulated wave from external equipment to be PSK-modulated.
The subcarrier modulator 104 controls the on/off state of the antenna
load selector switch 102 using a digital modulated signal generated from
sending data (TX_DATA). The subcarrier oscillator 103 generates a center
frequency Fs used to modulate the subcarrier. Thus, the subcarrier
modulator 104 can generate two modulated waves of an unmodulated carrier
having a center frequency Fo received by the antenna 100. The two
modulated waves respectively have center frequencies Fo+Fs and Fo-Fs
which are upwardly and downwardly apart by the subcarrier frequency Fs
from the unmodulated carrier at the center frequency Fo.

[0017]On the other hand, the band-pass filter 101 in the transmission
device 10 is used to extract only one of the two modulated waves (Fo+Fs
in the example shown). Thus, the same filter 101 passes the received
radio wave at the center frequency Fo and the modulated reflected wave at
the center frequency Fo+Fs. Here, the band-pass filter 101 need not be
used. In this case, however, the frequency spectrum of the modulated
reflected wave spreads out infinitely. Even when the band-pass filter 101
is used, it is unrealistic to hope for a frequency characteristic steep
enough to attenuate the side lobes. As a result, the spread of the
spectrum must be tolerated to a certain extent. Here, the center
frequency Fo of the unmodulated carrier is assumed to be in 2.4 GHz band,
and the subcarrier frequency Fs to be several tens of MHz.

[0018]On the other hand, the reader 11 includes an antenna 105, a sending
section 108 adapted to send the unmodulated carrier Fo, and a receiving
section 107 adapted to receive the modulated reflected wave Fo+Fs of the
unmodulated carrier Fo from the transmission device 10. The reader 11
further includes a circulator 106 adapted to separate the sending and
receiving sections 108 and 107 so as to simultaneously handle sending and
reception with a single antenna 105. The reader 11 still further includes
a baseband processing section 109 adapted to handle demodulation and
communication control. The unmodulated carrier at the center frequency Fo
is transmitted from the sending section 108 via the circulator 106 and
antenna 105. This unmodulated carrier returns to the reader 11 in the
form of a modulated wave at the center frequency Fo+Fs after being
reflected by the transmission device 10. The carrier is received by the
receiving section 107 via the antenna 105 and circulator 106. The carrier
is then demodulated by the baseband processing section 109 for use as
received data (RX_DATA).

[0019]Although mention was made of a unidirectional transmission from the
transmission device 10 to the reader 11, it is practically common that
control and user data is transmitted from the reader 11 to the
transmission device 10. However, the latter data transmission is not
directly related to the spirit of the present invention. Therefore, a
detailed description thereof will be omitted in the present
specification.

[0021]The system shown in FIG. 6 uses only one of the two modulated waves,
namely, the wave with the center frequency Fo+Fs. Therefore, the
band-pass filter 101 of the data transmission device 10 offers an
attenuation characteristic as denoted by reference numeral 203 to
attenuate the reflected wave signal. Thus, the same filter 101 passes the
unmodulated carrier 200 with the center frequency Fo and the reflected
wave 201 with the center frequency Fo+Fs. However, it is difficult to
implement the band-pass filter capable of steeply attenuating the side
lobes other than the main lobe of the reflected wave 201. As a result,
the side lobes spread out to a certain extent as illustrated in FIG. 7.

[0022][Patent Document 1]

[0023]Japanese Patent Laid-Open No. 2005-64822

SUMMARY

[0024]It is desirable to provide an excellent data transmission device
capable of high speed reflected wave data transmission at low power
consumption by performing a primary modulation adapted to modulate
sending data on a subcarrier followed by a secondary modulation adapted
to modulate the reflected wave of an unmodulated carrier from a data
reader.

[0026]It is desirable to provide an excellent data transmission device
capable of suppressing a modulated reflected wave from spreading into
side lobes during a primary modulation of a subcarrier.

[0027]A data transmission device according to the an embodiment for
sending back data on a reflected wave of an unmodulated carrier includes
an antenna. The antenna receives the unmodulated carrier Fo from a data
reader with which data is to be exchanged and sends a reflected wave of
the unmodulated carrier Fo to the data reader.

[0031]The data transmission device still further includes frequency mixing
means adapted to mix the unmodulated carrier Fo and modulated subcarrier
wave Fs to generate the new modulated waves Fo+Fs and Fo-Fs.

[0032]The RFID system is known as a communication system adapted to
wirelessly send data without having any own radio wave generating source.
A reflected wave transmission system, which is a type of the RFID system,
includes a data transmission device and data reader. The data
transmission device has a reflector adapted to receive an unmodulated
carrier, superimpose data on the reflected wave of the carrier and send
the modulated reflected wave. The data reader has a reflected wave reader
adapted to send an unmodulated carrier and read data from the modulated
reflected wave. The reflector demands no carrier generating source. This
ensures significantly reduced power consumption in data transmission,
thus providing an overwhelming superiority in performance over typical
wireless LANs.

[0033]There are several problems to be addressed in modulating a reflected
wave by changing the antenna load impedance of the reflector. Among such
problems are isolation between the sender and receiver in the data reader
and difficulties in providing faster data transmission due to the fact
that only slow modulation schemes such as ASK and PSK can be used. A
solution known to these problems is for the data transmission device to
modulate sending data on a subcarrier as a primary modulation, and then
modulate the reflected wave of an unmodulated carrier as a secondary
modulation.

[0034]In such a reflected wave transmission using primary digital
modulation, however, the antenna load selector switch is turned on and
off using a binary digital signal carried by the subcarrier. As a result,
only relatively slow subcarrier modulation schemes such as ASK, PSK, QPSK
and 8PSK, which demand only binary amplitude information, can be used.

[0035]In contrast, the data transmission device according an embodiment
does not rely on on/off operation of the antenna switch, namely, the
backscatter scheme, although modulating sending data on a subcarrier as a
primary modulation. Instead, this device relies on frequency mixing to
mix an unmodulated carrier and modulated subcarrier wave, thus modulating
the reflected wave and transmitting the modulated reflected wave back to
the data reader.

[0036]That is, the data transmission device according to an embodiment
mixes the unmodulated carrier Fo from the data reader and the analog
modulated subcarrier wave Fs for upconversion, thus generating the two
new modulated waves Fo+Fs and Fo-Fs. As described above, the data
transmission device can use such an analog modulated subcarrier wave. For
primary modulation, therefore, high speed modulation schemes can be used
including 16QAM, 64QAM and OFDM which carry binary or multi-level
amplitude information.

[0037]As a result, mobile devices such as digital camera and mobile phone,
demanded to be low in power consumption, offer high speed transmission of
moving and music data stored therein when equipped with the data
transmission device according to the present invention.

[0038]Further, the subcarrier modulation means of the data transmission
device according to an embodiment have a digital filter. This allows for
easy limitation of the bandwidth for subcarrier modulation. As a result,
the modulated waves Fo+Fs and Fo-Fs generated by the modulation are also
subjected to bandwidth limitation, thus reducing potential interference
with other systems.

[0039]Here, the frequency mixing means can be configured in the form of an
analog mixer using a diode whose operating point is set in the
non-saturated region. More specifically, the analog mixer has two input
ports, one for the analog modulated signal from the D/A conversion means,
and another for the antenna-received signal received via a band-pass
filter. A band-pass filter is provided between the antenna and frequency
mixing means to pass Fo and one of the Fo+Fs and Fo-Fs.

[0040]Alternatively, the frequency mixing means may include a splitter, an
FET mixer and a band-pass filter. The splitter divides the
antenna-received signal. The FET mixer mixes the analog modulated wave Fs
and antenna-received signal Fo divided by the splitter to generate
modulated reflected waves, namely, new modulated waves Fo+Fs and Fo-Fs.
The band-pass filter has the property to pass only one of the modulated
reflected waves Fo+Fs and Fo-Fs. The splitter sends one of the modulated
reflected waves Fo+Fs and Fo-Fs from the antenna. Here, the splitter may
include a 3-decibel coupler or Wilkinson divider.

[0041]Comparison of the two reflected wave modulation schemes, namely, the
scheme based on the above frequency mixing and that based on the existing
backscatter, in terms of power consumption, reveals that the former
cannot provide ultra-low power consumption in data transmission as the
latter can.

[0042]Therefore, the data transmission device according to an embodiment
may be configured to switch its reflected wave modulation scheme over to
the backscatter or frequency mixing scheme.

[0043]Backscatter modulation means include, for example, an antenna load
selector switch and band-pass filter. The antenna load selector switch
changes the antenna load impedance. The band-pass filter is disposed
between the antenna and antenna load selector switch and has the property
to pass the frequency Fo and one of the modulated reflected waves at the
frequencies Fo+Fs and Fo-Fs. The backscatter modulation means rely on the
change of antenna load impedance caused by the on/off operation of the
antenna load selector switch to modulate the reflected wave of the
antenna-received radio wave. The on/off operation of the switch is made
according to the digital modulated signal generated by the subcarrier
modulation means.

[0044]Further, an antenna switch is provided to connect the frequency
mixing means or backscatter modulation means to the antenna according to
the modulation scheme used by the subcarrier modulation means.

[0045]The subcarrier modulation means can use, according to the
communication quality, either a modulation scheme such as PSK, QPSK or
8PSK which demands only binary amplitude information, or a modulation
scheme such as 16QAM, 64QAM or OFDM which carries binary or multi-level
amplitude information. The antenna switch connects the backscatter
modulation means to the antenna when a modulation scheme, which demands
only binary amplitude information, is used by the subcarrier modulation
means. The antenna switch connects the frequency mixing means to the
antenna when a modulation scheme carrying binary or multi-level amplitude
information, is used by the subcarrier modulation means.

[0046]When the subcarrier modulation means use a slow modulation scheme
such as ASK, PSK, QPSK or 8PSK which demands only binary amplitude
information, the antenna switch is switched to the backscatter modulation
means. Modulation of the reflected wave using the backscatter scheme
ensures reduced average power consumption.

[0047]On the other hand, when the subcarrier modulation means use a fast
modulation scheme such as 16QAM, 64QAM or OFDM carrying binary or
multi-level amplitude information, the antenna switch is switched to the
frequency mixing means. Modulation of the reflected wave using the
frequency mixing scheme allows for the aforementioned fast primary
modulation.

[0048]The present application provides an excellent data transmission
device capable of high speed reflected wave data transmission at low
power consumption. The transmission device allows for such data
transmission by performing a primary modulation adapted to modulate
sending data on a subcarrier followed by a secondary modulation adapted
to modulate the reflected wave of an unmodulated carrier from a data
reader.

[0050]Still further, the present application provides an excellent data
transmission device capable of suppressing a modulated reflected wave
from spreading into side lobes during a primary modulation of a
subcarrier.

[0051]Additional features and advantages are described herein, and will be
apparent from the following Detailed Description and the figures.

BRIEF DESCRIPTION OF THE FIGURES

[0052]FIG. 1 is a view illustrating the configuration of a data
transmission device according to an embodiment;

[0053]FIG. 2 is a view illustrating the frequency spectrum of a reflected
wave transmission system using the data transmission device shown in FIG.
1;

[0054]FIG. 3 is a view illustrating the configuration of a data
transmission device 10 using an analog mixer which includes an FET or
transistor;

[0055]FIG. 4 is a view illustrating the frequency spectrum of a reflected
wave transmission system using the data transmission device shown in FIG.
3;

[0056]FIG. 5 is a view illustrating a configuration example of a data
transmission device configured so that the reflected wave modulation
scheme can be switched between the backscatter and frequency mixing
schemes;

[0057]FIG. 6 is a view illustrating a configuration example of a reflected
wave transmission system using a subcarrier; and

[0058]FIG. 7 is a view illustrating the frequency spectrum of the
reflected wave transmission system shown in FIG. 6.

DETAILED DESCRIPTION

[0059]The present application will be described below with reference to
the accompanying drawings according to an embodiment.

[0060]FIG. 1 illustrates the configuration of a data transmission device
according to an embodiment. The data transmission device shown in FIG. 1
has a reflector function adapted to send data through a reflected wave
transmission scheme. As a result, the same device is capable of high
speed reflected wave data transmission at low power consumption. To do
so, the same device performs a primary modulation adapted to modulate
sending data on a subcarrier followed by a secondary modulation adapted
to modulate the reflected wave of an unmodulated carrier from a data
reader. Mobile devices such as digital camera and mobile phone, demanded
to be low in power consumption, offer high speed transmission of moving
and music data stored therein when equipped with this data transmission
device. The configuration and processing operation of the data reader are
the same as those of the data reader shown in FIG. 6, and therefore a
description thereof is omitted.

[0062]The subcarrier modulator 302 performs a primary modulation adapted
to modulate sending data (TX_DATA) on the subcarrier frequency Fs, thus
generating a modulated subcarrier wave having a digital value (e.g.,
10-bit value) which is limited in bandwidth by a digital filter (not
shown). In addition to modulation schemes such as PSK, QPSK and 8PSK,
which demand only binary amplitude information, fast modulation schemes
such as 16QAM, 64QAM and OFDM carrying binary or multi-level amplitude
information can also be used. The D/A converter 303 converts the digital
modulated signal into analog form.

[0063]Reference numeral 300 denotes an analog mixer adapted to mix the
unmodulated carrier, received by the antenna 100, with the analog primary
modulated signal. In the example shown in the figure, the analog mixer
300 is a two-port mixer which includes a diode 305. The antenna 100 is
connected to the diode 305 via the band-pass filter (BPF) 101. On the
other hand, a DC bias voltage Vb is applied to the diode 305 via a
resistor R1 (308) and inductor L1 (306), thus setting the operating point
of the diode 305 in the non-saturated region.

[0064]The analog modulated wave at the center frequency Fs from the D/A
converter 303 is sent to the diode 305 via an inductor L2 (310). The
analog modulated wave is mixed with the unmodulated carrier Fo from the
antenna 100 in the non-linear region of the diode 305, thus producing two
modulated waves respectively at the center frequencies Fo+Fs and Fo-Fs.
The band-pass filter 101 has the property to pass Fo and Fo+Fs. As a
result, the antenna 100 sends back the modulated wave Fo+Fs.

[0065]It should be noted that capacitors C1 (304) and C2 (309) are used to
cut off DC component resulting from the DC bias voltage Vb contained in
the analog modulated wave at the center frequency Fs and the newly
generated modulated waves Fo±Fs. A pair of the inductor L1 (306) and a
capacitor C3 (307) and another pair of the inductor L2 (310) and a
capacitor C4 (311) each function as a low-pass filter (LPF) adapted to
pass DC and the subcarrier frequency Fs while presenting a high impedance
to the frequency Fo. These components are used to reduce leakage of the
unmodulated carrier at the frequency Fo, received by the antenna 100, to
components other than the diode 305.

[0066]As described above, an analog mixer which includes a two-port diode
mixer can generate the modulated wave with the center frequency Fo+Fs and
transmit the wave to the data reader.

[0067]FIG. 2 illustrates the frequency spectrum of a reflected wave
transmission system using the data transmission device shown in FIG. 1.
In FIG. 2, reference numeral 200 denotes an unmodulated carrier from the
data reader. We assume that the data reader is the same as that shown in
FIG. 6.

[0068]In FIG. 2, reference numerals 310 and 311 denote the two modulated
waves Fo+Fs and Fo-Fs generated by the data transmission device 10,
respectively. In the example shown in this figure, only the modulated
wave at the center frequency Fo+Fs is used. Therefore, the band-pass
filter 101 has the attenuation property as denoted by reference numeral
312. As a result, the same filter 101 passes the unmodulated carrier 200
at the center frequency Fo and the modulated wave 310 at the center
frequency Fo+Fs.

[0069]Further, the modulated wave is limited in bandwidth by a digital
filter (not shown) provided in the subcarrier modulator 302. As a result,
the modulated wave spectra 310 and 311 do not spread over a wide
bandwidth. This makes it possible to transmit only the main lobe as
illustrated in FIG. 2.

[0070]In the embodiment shown in FIG. 1, the data transmission device 10
employs a mixer which includes a diode for use as an analog mixer adapted
to mix the unmodulated carrier received by the antenna 100 with the
analog primary modulated signal. However, the diode mixer is generally
known to cause a large loss. Therefore, the inventors propose a
configuration of the data transmission device 10 as shown in FIG. 3 as an
alternative embodiment. The data transmission device uses an analog mixer
which includes an FET or transistor.

[0072]The subcarrier modulator 302 performs a primary modulation adapted
to modulate sending data (TX_DATA) on the subcarrier frequency Fs, thus
generating a modulated subcarrier wave having a digital value (e.g.,
10-bit value) which is limited in bandwidth by a digital filter (not
shown). In addition to modulation schemes PSK, QPSK and 8PSK, which
demands only binary amplitude information, fast modulation schemes such
as 16QAM, 64QAM and OFDM carrying binary or multi-level amplitude
information can also be used. The D/A converter 303 converts the digital
modulated signal into analog form.

[0073]Reference numeral 400 denotes a mixing section adapted to mix the
unmodulated carrier received by the antenna 100 with the analog primary
modulated signal. In the example shown in FIG. 3, the mixing section 400
includes a splitter 401, mixer 402 and low-pass filter 404.

[0074]The antenna 100 is connected to a terminal `s` of the splitter
(combiner) 401. The signal received by the antenna 100 is divided into
two and delivered to terminals `a` and `b.` A 3-decibel coupler or
Wilkinson divider is used as the splitter 401. The loss caused by the
division is 3-decibel.

[0075]The mixer 402 includes an FET. The mixer receives, as an input, the
output of the terminal `a` to which one of the signals generated by the
splitter 401 is delivered. This input is fed through a local input
terminal. The mixer receives, as another input, an analog modulated
subcarrier wave from the primary modulation section. This input is fed
through an IF input terminal. The mixer mixes the two inputs for
upconversion. The mixer outputs the two modulated waves at the center
frequencies Fo+Fs and Fo-Fs from its RF output terminal.

[0076]The low-pass filter 404 has the property to pass only the wave Fo+Fs
of the two. The output of the same filter 404 is connected to the
terminal `b` of the splitter (combiner) 401. The antenna 100 sends back
the modulated wave Fo+Fs.

[0077]The mixer 402 typically has a conversion gain. However, a total loss
of 6-decibel occurs between the terminals `s` and `a` and between the
terminals `b` and `s.` Therefore, an amplifier 403 may be provided after
the mixer 402 in some cases.

[0078]As described above, the three-port FET mixer can generate the
modulated wave at the center frequency Fo+Fs, which can then be sent back
to the data reader.

[0079]FIG. 4 illustrates the frequency spectrum of a reflected wave
transmission system using the data transmission device shown in FIG. 3.
In FIG. 3, reference numeral 200 denotes an unmodulated carrier from the
data reader. We assume that the data reader is the same as that shown in
FIG. 6.

[0080]Also, in FIG. 4, reference numerals 310 and 311 denote the two
modulated waves Fo+Fs and Fo-Fs generated by the data transmission device
10, respectively. In the example shown in FIG. 4, only the modulated wave
at the center frequency Fo+Fs is used. Therefore, the band-pass filter
101 has the attenuation property as denoted by reference numeral 500. As
a result, the same filter 101 passes the modulated wave 310 at the center
frequency Fo+Fs.

[0081]Further, the modulated wave is limited in bandwidth by a digital
filter (not shown) provided in the subcarrier modulator 302. As a result,
the modulated wave spectra 310 and 311 do not spread over a wide
bandwidth. This makes it possible to transmit only the main lobe as
illustrated in FIG. 4.

[0082]As described above, the data transmission device shown in FIGS. 1
and 3 performs conversion of a digital modulated signal into analog form,
followed by mixing of the modulated wave with an unmodulated carrier
using an analog mixer, namely, frequency mixing, thus generating
modulated reflected waves. In such a case, a primary modulation, which
carries binary or multi-level amplitude information, provides faster
reflected wave data transmission.

[0083]Here, we will compare the two reflected wave modulation schemes, one
based on frequency mixing as shown in FIGS. 1 and 3 and the other based
on backscatter as shown in FIG. 6, in terms of power consumption.

[0084]The latter scheme based on backscatter relies on the change of
antenna load impedance caused by the on/off operation of the antenna
switch to modulate the reflected wave of the antenna-received radio wave.
The on/off operation of the switch is made by a digital modulated signal.
If a gallium arsenide FET switch is used as the antenna switch, the
current demanded for this modulation is almost 0 mA. This ensures
ultra-low power consumption in data transmission.

[0085]In contrast, according to the former scheme based on frequency
mixing, the current consumption using a diode mixer as shown in FIG. 1 is
about 1 mA. The current consumption of an FET mixer shown in FIG. 3 is
about 5 mA. Although low, these current consumptions are higher than that
using a gallium arsenide FET switch. As a result, ultra-low power
consumption cannot be ensured in data transmission.

[0086]For this reason, the inventors further propose a data transmission
device configured to switch the reflected wave modulation scheme between
the backscatter and frequency mixing schemes. In this case, the data
transmission device supports a plurality of modulation schemes for
primary subcarrier modulation. In addition, the data transmission device
adaptively switches the reflected wave modulation scheme to the
backscatter or frequency mixing scheme when switching the subcarrier
modulation scheme according to the communication quality. For example,
when a slow modulation scheme such as ASK, PSK, QPSK or 8PSK is used
which demands only binary amplitude information, the average power
consumption can be reduced by using the backscatter scheme for modulation
of the reflected wave.

[0087]FIG. 5 illustrates the configuration of the data transmission device
10 used in this case.

[0089]The subcarrier modulator 302 performs a primary modulation adapted
to modulate sending data (TX_DATA) on the subcarrier frequency Fs, thus
generating a modulated subcarrier wave having a digital value (e.g.,
10-bit value) which is limited in bandwidth by a digital filter (not
shown). In addition to modulation schemes such as PSK, QPSK and 8PSK,
which demand only binary amplitude information, fast modulation schemes
such as 16QAM, 64QAM and OFDM carrying binary or multi-level amplitude
information can also be used. The D/A converter 303 converts the digital
modulated signal into analog form.

[0090]The data transmission device illustrated in FIG. 6 further includes
two secondary modulation sections, one based on the backscatter scheme
and another based on the frequency mixing scheme. One of these two
modulation sections is alternatively connected to the antenna 100.

[0091]The former secondary modulation section based on the backscatter
scheme includes the band-pass filter (BPF) 101 and antenna load selector
switch 102.

[0092]One end of the antenna load selector switch 102 is grounded. The
same switch 102 serves as a load of the antenna 100 and is
short-circuited when turned on and open-circuited when turned off, thus
allowing an unmodulated reflected wave received by the antenna 100 to be
modulated by PSK. The subcarrier modulator 104 controls the on/off state
of the antenna load selector switch 102 using a digital modulated signal
generated from sending data (TX_DATA).

[0093]On the other hand, the band-pass filter 101 is used to extract only
one of the two modulated waves (Fo+Fs in the example shown). Thus, the
same filter 101 passes the received radio wave at the center frequency Fo
and the modulated reflected wave at the center frequency Fo+Fs.

[0094]Thus, the secondary modulation section based on the backscatter
scheme can generate two modulated waves for the unmodulated carrier at
the center frequency Fo received by the antenna 100. The two modulated
waves respectively have the center frequencies Fo+Fs and Fo-Fs which are
upwardly and downwardly apart by the subcarrier frequency Fs from the
unmodulated carrier at the center frequency Fo.

[0095]On the other hand, a secondary modulation section 601 based on the
latter frequency mixing scheme mixes the frequencies of two signals. One
of the two signals is an analog modulated signal generated by converting
the output of the subcarrier modulator 104 into analog form with the D/A
converter 303. The other signal is an unmodulated wave received by the
antenna 100. The secondary modulation section 601 performs the above
frequency mixing to modulate the unmodulated reflected wave. The same
section 601 is configured in the same manner as a functional block
denoted by reference numeral 300 in FIG. 1 or that denoted by reference
numeral 400 in FIG. 3. Therefore, a description thereof will be omitted.

[0096]An antenna switch 600 is adaptively switched to either of the two
secondary modulation sections according to the modulation scheme (MOD)
used by the subcarrier modulator 302.

[0097]More specifically, when a slow modulation scheme such as ASK, PSK,
QPSK or 8PSK is used which demands only binary amplitude information, the
antenna switch 600 is switched to an "A" position. Modulation of the
reflected wave using the backscatter scheme ensures reduced average power
consumption.

[0098]On the other hand, when a fast modulation scheme such as 16QAM,
64QAM or OFDM is used which carries binary or multi-level amplitude
information, the antenna switch 600 is switched to a "B" position. As a
result, the above high speed primary modulation can be made possible by
modulating the reflected wave by the frequency mixing scheme.

[0099]It should be noted that the frequency spectra of the reflected wave
transmission system are as illustrated in FIG. 7 and FIG. 2 or 4,
respectively when the antenna switch 600 is set to the "A" and "B"
positions. Therefore, a description thereof is omitted.

[0100]It should be understood that various changes and modifications to
the presently preferred embodiments described herein will be apparent to
those skilled in the art. Such changes and modifications can be made
without departing from the spirit and scope of the present subject matter
and without diminishing its intended advantages. It is therefore intended
that such changes and modifications be covered by the appended claims.